The present invention relates to transverse-mode stabilized laser diodes, and more particularly to ridge stripe AlGaInP visible light laser diodes with oscillation wavelength not greater than 680 nm.
AlGaInP visible light laser diodes are attractive light sources for optical information processing systems such as laser printers, barcode readers, and optical disc systems. A conventional transverse-mode stabilized AlGaInP visible light laser diode is shown in FIG. 1 as reported in the Abstract of Papers for '87 Fall Meeting of the Japan Society of Applied Physics, p. 764, 19a-ZR-5.
Referring to FIG. 1, an n-GaAs buffer layer 2 is formed on an n-GaAs substrate 1. On top of the buffer layer 2 there is formed a double heterojunction structure consisting of an n-AlGaInP cladding layer 3, a GaInP active layer 4, a ridge stripe p-AlGaInP cladding layer 5, a p-GaInP cap layer 6, a pair of n-GaAs current blocking layers 7 and a p-GaAs contact layer 8.
Here, the fabrication step of the conventional structure shown in FIG. 1 will be described. First, a five-layer structure from the n-GaAs buffer layer 2 to the p-GaInP cap layer 6 is formed sequentially by a first vapor phase growth. Subsequently, a stripe-shaped mask using a 5 .mu.m-width SiO.sub.2 film is formed on the cap layer 6 by photoetching, and a ridge stripe structure is formed by etching the p-AlGaInP cladding layer 5 halfway. Next, the n-GaAs current blocking layers 7 are formed selectively on the mesa part except for the stripe-shaped SiO.sub.2 film mask by a second vapor phase growth. Then, after the removal of the SiO.sub.2 film mask, the p-GaAs contact layer 8 is grown all-over the surface by a third vapor phase growth, a p-side ohmic electrode 9 of AuZn/Au is formed on the contact layer 8 and an n-side ohmic electrode 10 of AuGe/Au is formed on the GaAs-substrate 1.
In this prior art structure, the current contraction is achieved by the n-GaAs current blocking layers 7. Further, the p-GaInP cap layer 6 plays the role of preventing the increase of the electrical resistance caused by the band discontinuity between the p-AlGaInP cladding layer 5 and the p-GaAs contact layer 8.
On the other hand, the transverse-mode stabilization is achieved by the formation of a refractive index distribution in the active layer 4 along a function plane in the direction perpendicular to an optical axis of the laser diode by utilizing the light absorption loss caused by the n-GaAs current blocking layers 7 at thinned peripheral portions of the p-AlGaInP cladding layer 5.
In recent years there are increasing demands for refractive index guided type laser diodes that can be operated in the fundamental transverse mode and has a low oscillation threshold and a low astigmatism.
However, in the conventional refractive index guided type laser diodes as described above, since the built-in effective refractive index distribution in the active layer 4 along a junction plane is formed by the light absorption loss caused by the n-GaAs current blocking layer 7, the oscillation threshold current is high and the astigmatism is a relatively large value of 10-13 .mu.m, leading to a drawback that it is not easy to narrow down the laser beam to a minute spot.
Moreover, due to the necessity for making the thickness of the p-AlGaInP cladding layer 5 at both sides of the ridge or mesa portion to a small value in the range of 0.1 to 0.2 .mu.m, the active layer 4 under both sides of the mesa portion undergoes a thermal damage during the formation of the n-GaAs current blocking layer 7 by a second vapor phase growth, and thus the life-time of the laser diodes is shortened.